I'm getting ready to epoxy coat the garage floor and am trying to completely prep a few oil spots. Most of the oil is gone, but water still beads slightly. I've tried Simple Green, and I used a power washer on the weekend. I'm looking for something a bit stronger. Any recommendations?

Have you tried muriatic acid to see if that cleans it enough? You have to use that before you paint anyway.

I think that's all I did when I did my cheapy paint job 10 years ago. I may have used some lacquer thinner on the tough spots, but that was about it, IIRC.

I washed mine with water and detergent first to get the main things cleaned... with a fairly abrasive brush
Then I washed the whole thing w/ muriatic (or something like that) acid.
You have to let the acid work its way on the cement... and pass the abrasive brush as the acid is working.
Make sure it's ventilated... the acid smells great.
Wear some clean rain boots, or fishing boots.
Once that is done, rinse thoroughly and let it dry.

another advice.
the cleaning product that comes in the kits ain't enough... so get some muriatic acid to do the cleaning. you can always use what's in the kit after you've cleaned it... i did it... probably overkill... but that's me.
Also, get an extra kit based from the #s given.
If you can get extra flakes, get them.

Brake clean. It's a great way to disolve oil and if it's just for a few spots it won't send you to the porr house. Spray it on let it work for a few seconds then wipe up the majority. While the spot is still wet hit it with an ordinary soap and water type cleaner.

Thanks guys. I used some powder stuff yesterday, scrubbed it, hosed it....seems to have done the trick. The kit does come with muriatic acid, but I didn't want that to be the the only prep. This stuff from yesterday seemed to have cleaned the driveway a bit too much (even in it's highly diluted states) as I hosed it out.